The present invention relates to an optical microphone or transducer including a light source, a fibre optic cable and a Fabry-Perot interferometer equipped with two reflectors, wherein one end face of the fibre optic cable forms a first reflector and the second reflector is arranged at a distance therefrom and is displaceable or deflectable by fluctuations in the sound air pressure.
In many fields of application, it is necessary to arrange the microphone well away from the associated audio amplifiers. The unamplified electrical microphone signals then have to traverse long paths. Cable losses and capacitances as well as stray electromagnetic pick-up in the cable limit the length of the connecting cable to the electronic amplifier. Consequently, if distances in the order of kilometers have to be traversed, for example for traffic monitoring purposes, then the electrical signal is frequently initially digitised and, following an electro-optic conversion, is transmitted as an optical signal over a glass fibre cable to a distant receiver in order to ensure optimum transmission quality. It is a disadvantage here that the electro-optic conversion requires a costly processing of the electrical signal at or near the place where the microphone is situated. Thus, in addition to the optical test signal lead, an electrical current supply is needed at the place where the microphone is situated. The electronic equipment located there is subject to faults and has to be maintained.
Sound induced fluctuations in the air pressure can, however, be directly converted into a phase modulation of a light wave and then into an intensity modulated optical signal by a process of superimposition. An optical microphone of this type can thus transmit the test signal over a glass fibre cable without using an electro-optical conversion process. The optical signal can be amplified and processed at the test analysis location using conventional electronic equipment following an opto-electric conversion process. An optical microphone is thus immune to electromagnetic interference. Furthermore, the problems with earth loops, which occur with electrical installations, especially when long transmission paths are involved, are eliminated.
A microphone of this type has been described by H. Naono, M. Matsumoto, K. Fujimura, K. Hattori in the Proc. 9th Int. Conf. on Optical Fibre Sensors (OFS-9), Florence 1993, pages 155-158 under the title "Fibre-Optic Microphone using a Fabry-Perot interferometer". The optical microphone described therein includes a miniature Fabry-Perot Interferometer as the sensing element. A highly coherent laser diode is used as the light source. The pencil of light emerging from a single-mode glass fibre is reflected by a reflectively coated film diaphragm disposed some 10 .mu.m to 100 .mu.m away and then coupled back into the fibre. Sound induced fluctuations in the air pressure cause in-phase alterations of the glass fibre--diaphragm spacing. The correspondingly phase modulated (.DELTA..PHI.(L), L=Fabry-Perot length), reflected light wave is superimposed on the light wave that is partially reflected at the glass fibre--air interface to form an interference signal (intensity I=2 R (1-cos .DELTA..PHI.), where the specular reflectance R&lt;&lt;1) which is conducted in the form of an acoustic frequency intensity modulated light wave over the glass fibre feeder to a photo-detector where an opto-electric conversion process takes place so that processing in a conventional manner can then be effected. By virtue of the interference-free superimposition of the forward and return light waves, the optical microphone only requires just one single-mode glass fibre cable, which is simultaneously used as a "supply-" and as a signal lead, for establishing the connection between the test location and the processing location.
The known construction nevertheless has some disadvantages. The cos characteristic of the interference signal (only at low reflectances R; the cos characteristic changes into an Airy function at higher values of R) can lead to temperature induced signal fading due to the thermal expansion of the Fabry-Perot resonator: the small signal sensitivity (increase of the cos .DELTA..PHI.characteristic) approaches zero at the maximum or minimum of the cos function. For the purposes of stabilising the small signal sensitivity, costly demodulation and stabilising processes, which are known as homodyne and heterodyne processes, are required. In the case of the cited reference, the working point is stabilised by actively de-tuning the wavelength of a DFB laser diode. The use of laser diodes as the highly coherent light sources usually employed for interferometry requires costly temperature and current stabilisation processes as well as an effective method of cutting off the light reflected back by the sensor into the diode thus resulting in correspondingly high costs for the whole system. A further problem arises in the case of the cited arrangement when it is being used outside due to the necessarily very thin (3 .mu.m) and sensitive foil diaphragm that is required for the sound induced phase modulation process.